Hydraulic brake boosters have been designed to provide an assist in the actuation of a master cylinder to pressurize fluid to developed a force to effect a brake application. In order to reduce the cost of a hydraulic brake booster, often the same hydraulic power source used to supply a steering gear is used to power a hydraulic brake booster. The controls for such hydraulic brake boosters are designed such that a minimum amount of hydraulic fluid is always available for operation of either the hydraulic brake booster or the steering gear. In certain brake boosters, of a type disclosed in U.S. Pat. Nos. 3,967,536; 4,131,055; 4,179,980; 4,514,981; 4,724,674 and 5,442,916, the booster operates by restricting flow from one side of a power piston to the other side of the power piston to create a fluid pressure differential which causes the power piston to move and provide power assisted displacement of the pistons in a master cylinder. In this type of brake booster, the master cylinder and booster are joined together and as a result the overall length occupies considerable under hood space of a vehicle. Because of the efficiency of such brake boosters they have found application in many vehicles and in particular van and certain mid-sized trucks. However, in some models of recently manufactured vehicle, the physical design of the under hood space is often restricted or reduced, and as a consequently locating a brake booster and other components is often a difficult task. To better utilize under hood space, it has been disclosed in U S. Pat. Nos. 5,329,769, 5,313,796 and U.S. patent application Ser. No. 09/097,778, now U.S. Pat. No. 6,038,857 that certain brake systems components could be located remotely from under the hood. These brake systems functioning in an adequate manner but require a considerable number of components in the control apparatus to provide a stable and smooth application of the wheel brakes.
U.S. Pat. No. 4,865,399 discloses an anti-lock brake system wherein pressurized fluid developed by a pump system is supplied to wheel brakes to effect a brake application. The time the pressurized fluid is supplied to any individual wheel brake is alternately increased and decreased through the actuation of a solenoid valve by a pulse-width-modulated signal to produce a desired braking deceleration for a vehicle.